AMD 64 SysV calling convention

                  1    2    3    4   5   6
Syscall    RAX (RDI, RSI, RDX, R10, R8, R9) -> RAX
Function       (RDI, RSI, RDX, RCX, R8, R9) -> RAX


dietlibc implements some time-related syscalls via VDSO on x86_64.
This is not done in the bundled libc, the implementation seems to be fragile
and init is not the kind of process to benefit much from using VDSO.
